1. Technical Field
The present invention relates to a light source device and a projector.
2. Related Art
In recent years, while demands for downsizing of projectors are growing, in accordance with increase in output power of semiconductor lasers and appearance of blue semiconductor lasers, projectors using laser beam sources have been developed. The projectors of this kind are capable of having a sufficiently large color reproduction range because the wavelength band of the light source is narrow, and of downsizing and reducing the components, and consequently, have a great potential as display devices of the next generation. In this case, laser beam sources of the three colors, red (R), green (G), and blue (B) are required as the light source. Since, for example, the fundamental vibrations for the R-light source and the B-light source can be obtained by semiconductor lasers while the fundamental vibration for the G-light source cannot be obtained by a semiconductor laser, use of second harmonic generation (hereinafter abbreviated as SHG), the technology of generating a second harmonic wave by making an infrared beam from an infrared laser enter a nonlinear optical element, has been considered.
In the wavelength conversion of light using the nonlinear optical effect, it is required that the phase matching condition is fulfilled between the fundamental wave before the conversion and the harmonic wave after the conversion, and consequently, a quasi-phase matching method for periodically inverting the polarization direction in the crystal is used therefor. In general, a structure (hereinafter referred to as a periodic polarization inversion structure in the present specification) having the polarization direction periodically inverted with a fine pitch is formed in the crystal of MgO:LiNbO3 to form a wavelength conversion element. However, the actual wavelength conversion element has an extremely narrow allowable range of wavelength satisfying the phase matching condition, and the output (conversion efficiency) is significantly lowered when the wavelength of the fundamental wave is shifted, if only a little. On the other hand, it is known that the conversion efficiency strongly depends on the temperature of the wavelength conversion element. There has been proposed a laser beam source device, which uses this dependency, has a plurality of wavelength conversion elements for executing wavelength conversion on a plurality of laser beams, and individually controls the temperature of each of the wavelength conversion elements, thereby keeping the conversion efficiency of the overall wavelength conversion elements (see e.g., JP-A-2006-352009 (Document 1)).
Further, there has been proposed a configuration for controlling the temperature of a nonlinear optical crystal by applying flow of a liquid or a gas around the nonlinear optical crystal (see e.g., JP-A-5-100267 (Document 2)). In an optical harmonic wave generation device described in the Document 2, two or more nonlinear optical crystals are arranged in series in the proceeding direction of the light, and the second harmonic generation is obtained by a laser beam sequentially passing through the nonlinear optical crystals. In the optical harmonic wave generation device, an inflow section and an outflow section of a medium are disposed in the vicinity of the nonlinear optical crystal disposed at the center of the three nonlinear optical crystals, and the medium flow is applied in a direction perpendicular to the arranging direction of the plural nonlinear optical crystals, thereby making the temperature of the nonlinear optical crystals constant.
In the past, ordinary projectors have often used a discharge lamp such as a super high-pressure mercury lamp as a light source. However, the discharge lamp of this kind has had problems of a relatively short life, a difficulty in quick lighting, a small color reproduction range, and so on. In contrast, according to the projector using the laser beam source described in the Document 1 or 2 above, the problems described above can be solved. However, in the technology described in the Document 1 or the Document 2, the projection light on the screen by the laser beam source has the phases of the light beams aligned with each other in the adjacent areas, and consequently has extremely high coherency. Since the coherent length of a laser beam extends to several tens meters in some cases, if two or more laser beams are combined, the beams combined through the light paths having a shorter difference than the coherent length inevitably cause strong interference. Therefore, a sharper scintillation (interference pattern) than that of a super high-pressure mercury lamp appears to cause significant degradation in display quality.
Further, in the configuration of cooling two or more nonlinear optical crystals described in the Document 2, there is a possibility that the flow of the medium in the vicinity of the nonlinear optical crystal disposed on an end portion of the two or more nonlinear optical crystals might be blocked, and therefore it is difficult to keep the two or more nonlinear optical crystals at predetermined temperature. As a result, the conversion efficiency of light in the nonlinear optical crystals is problematically lowered.